A transfer to Manchester City for Gianluigi Donnarumma seems almost certain, but if you look at the Turkish media, they are still saying he will come to Galatasaray. 
Apparently, Manchester City will pay 50 million Euro for Donnarumma. And then they may sell Ederson. Galatasaray could then acquire Ederson. The likelihood of Donnarumma coming to Türkiye was already very low. Why would he go to a league outside the top five when he could join one of the biggest team in the Premier League? It's still too early for him.
I completely agree with your analysis that it makes much of sense for Gianluigi Donnarumma to join top team like Manchester City to compete for big trophies making reports from Turkish media about him going to Galatasaray seem like wishful thinking. Move to league outside of Europe top five would be step down for him at this stage in his career. If Manchester City does spend much of money to buy Donnarumma, they would likely sell their current goalkeeper Ederson, which could be great opportunity for Galatasaray to sign very good player. Apparently, Manchester City will pay 50 million Euro for Donnarumma. And then they may sell Ederson. Galatasaray could then acquire Ederson. The likelihood of Donnarumma coming to Türkiye was already very low. Why would he go to a league outside the top five when he could join one of the biggest team in the Premier League? It's still too early for him.